It does have me wondering if I can take back my 10.5.0 installer that shipped via Up-to-Date and exchange it for a 10.5.1 DVD. There's a pretty good argument to be made that it's defective and should be covered under AppleCare for the systems I purchased in October, and the "Genius Bar" employees are usually dumb enough that it might work. Of course, I could luck out and actually get a sincerely intelligent and reasonable "Genius Bar" employee, but why buck the trend?
But they might just say "has 10.5.0 broken anything?" if the answer is no then your argument would collapse. However every OS has upgrades and that is the price of every new system..
But 10.5.1 should have been the GM, 10.5.0 was clearly rushed out the door to save another delayed embarrassment.